Thin dielectric layers inside hollow metallic waveguides are used to improve the waveguide transmission characteristics as the dominant waveguide mode changes into the hybrid ${\mathrm{HE}}_{11}$ mode. We investigate the effect of 1 μm thick silver iodide (AgI) coatings on the fundamental modes in cylindrical waveguides at terahertz (THz) frequencies, in the regime of the dielectric layer being thinner than the optimal thickness $h_{\text{opt}}(2\mathrm{THz})\approx 20\mathrm{\mu m}$. In the region of 1–3.2 THz, the lowest-order modes are similar in profile to the ${\mathrm{TE}}_{11}$ and ${\mathrm{TM}}_{11}$ modes, as determined by the time-resolved near-field measurements and verified numerically. Higher-order modes are detected experimentally as mode mixtures due to the multimode propagation. Numerical electromagnetic modeling is applied to resolve the mode structure ambiguity, allowing us to correlate experimentally detected patterns with a superposition of the ${\mathrm{TM}}_{11}$ and the higher-order mode, ${\mathrm{TE}}_{12}$. Mode profiles determined here indicate that in the regime of ultrathin dielectric ($h\ll 0.1{\lambda }_{\text{eff}}$), the dielectric layer does not transform the dominant mode into the low-loss ${\mathrm{HE}}_{11}$ mode. Experimental mode patterns similar to the ${\mathrm{HE}}_{11}$ and the ${\mathrm{TE}}_{01}$ modes nevertheless can be formed due to mode beating. The results indicate that the Ag/AgI waveguides can be used for guiding THz waves in the ${\mathrm{TE}}_{01}$ mode or the ${\mathrm{TE}}_{12}$ mode with high discrimination against other modes.